This invention relates to steering systems for a vehicle, and more particularly, the invention relates to a steering system designed to address Ackerman errors.
Vehicle steering systems steer wheels on opposing sides of the vehicle in response to a steering input. Typically, an input from a steering wheel is directed through mechanical steering components to steer the wheels on each side. More specifically, the steering wheel is connected to a control box which is connected to one or more mechanical linkages connecting the opposing wheels. The mechanical linkages and control boxes are difficult to package in that they must be routed from one of the side to the other to mechanically connect the wheels.
During a vehicle turn, the inner wheels travel in a path having a smaller radius than the outer wheels. As a result, the inner wheel must be turned a greater amount than the outer wheel so that the wheels travel along their respective radial paths. Due to the limitations of the mechanical linkage provided by the steering system, the angle of the inner and outer wheels may not be changed independently from one another. The angles of the wheels during a turn is commonly referred to as the Ackerman angle. Accordingly, steering systems are designed so that the wheels are pointed in the direction of their respective radial paths for one particular turn, preferably in the range of 20xc2x0 to 30xc2x0, such that there is no Ackerman error. For all other turning angles an Ackerman error exists such that at least one of the wheels is not pointed in the direction of its radial path. Ackerman error essentially causes the wheels to pull the vehicle in slightly different directions and accelerates tire wear. Therefore, what is needed is a steering system that eliminates Ackerman error for all of the vehicle turning angles to extend tire life.
The present invention provides a steering system for a vehicle that includes a steering knuckle which is rotatable about an axis. The steering knuckle includes upper and lower portions or attachment points that define the steer axis. A lower suspension interconnects the lower portion to the vehicle frame or body. An energy absorbing mechanism, preferably including an inner spring and a shock absorber, interconnects the upper portion to the vehicle body or frame. A rotational drive unit is connected to the steering knuckle for rotating the steering knuckle about the axis. A control module receives a steering input corresponding to the desired Ackerman angle and commands the rotational drive unit to rotate the steering knuckle about the axis to the desired Ackerman angle. Rotational drive units for each side of the vehicle ensure that the wheels are turned to the appropriate Ackerman angle for each wheel. The rotational drive unit may include an electric motor, or a pneumatic or hydraulic pump. In one preferred embodiment, the drive unit includes a rotary actuator which has a shaft connected rigidly to the steering knuckle. The actuator shaft may consist of either a single vane or multiple vanes. The fluid pump pumps fluid into the housing to rotate the vane with the shaft and steering knuckle about the axis in response to a command from the control module.
Accordingly, the above invention provides a steering system that eliminates Ackerman error for all of the vehicle turning angles and extends tire life.